Collagen-induced arthritis (CIA) is an experimental model of autoimmune arthritis induced by immunization of susceptible strains of mice with type II collagen (CII). Our recent studies using HLA-DR transgenic (Tg) mice expressing HLA class II molecules associated with susceptibility to rheumatoid arthritis (RA) reveal that DR1 (DRB1*0101) and DR4 (DRB1*0401) can bind and present peptides derived from human (h) CII, and that mice bearing these transgenic DR molecules are susceptible to CIA. The immune response to hCII and arthritis can be down regulated by an analog peptide of CII, A12, CII 256-276 (F263->N, E266->D), when it is co-administered at the time of immunization with hCII. Human CII- sensitized spleen and lymph node cells from DR Tg mice produce increased amounts of IL-4 and IL-10 when cultured with Al2 in comparison to cells cultured with the wild-type CII peptide (CII 256-276). These observations suggest that the suppressive effect of A12 on immune response to CII and CIA in these DR Tg mice is mediated by a shift in the cytokine profile, from that of a Thl to a Th2. However, the mechanism(s) whereby the shift is brought about is not known. Al2 contains two amino acid substitutions as compared to the wild-type CII peptide. The substitutions are of two types, a) residue 263 that participates in peptide binding to the DR molecule, and b) residue 266 that interacts with the TCR. The hypothesis to be tested in this application is that the A12 effect on CIA is mediated through polarization of the specific immune response to a predominant Th2 profile, and that this polarization is caused by changes in the affinity between either DR and peptide or DR/peptide interaction with the TCR leading to altered T cell signaling and, consequently, the production of cytokines. We, therefore, plan to: 1) Identify the structural characteristics of the analog peptide Al2 that mediate its modulation of the immune response to CIIl and CIA in DR Tg mice; 2) Determine whether encoding the Al2 substitutions within the triple helical CII molecule increases the efficacy in inhibiting the autoimmune response in CIA; 3) Determine whether the suppression of CIA produced by A12 administration is dependent on IL-l0 or IL-4 secretion; and 4) Determine the mechanism by which Al2 alters T cell function by analyzing signaling pathways involved in T cell activation. Information gained from these studies will provide important insight for the design of novel therapeutic approaches that may prove beneficial in the treatment of autoimmune arthritis in humans.